Ron,
I have one such boiler experience, reducing a 4-5/8" barrel diameter to 4-1/8" diameter in a 1" scale Atlantic. It was reported to me that the boiler steamed like a demon and there was no apparent adverse effect from reducing the barrel diameter. However, I don't recall what the cylinder diameter was and it seems to me 1.5" diameter cylinders are going to want all they can get out of a 4" boiler, especially if worked hard. But based on the one experience I had, if you maintain roughly the same amount of flue heating surface, under normal operating conditions the reduced volume wouldn't be missed.
The impact of such a change will be in three areas. The first will be externally which can be handled with built-up insulation. The second will be that the internal volume of the boiler, the water and steam storage space, will be proportionately reduced. Thirdly, it's possible that the number and/or of flues might be reduced thus reducing the heating surface which will alter the steaming capability of the boiler. Whether the impact is great enough to require a design change in some other area, I don't know, although both conditions can be overcome by an increased firing rate.
PS - Hopefully your scrounged piece of copper is new material. A little oxide on an old piece of unused pipe is OK, but salvaged scrap is not a good thing to use in a boiler.